Eddy current testing is a non-destructive test technique based on inducing electrical currents in the material being inspected and observing the interaction between these currents and the material. Eddy currents are generated by electromagnetic coils in the test probe, and monitored simultaneously by measuring probe electrical impedance. Since it is an electromagnetic induction process, direct electrical contact with the sample is not required; however, the sample material must be electrically conductive.
When inspecting for defects, it is essential that flow of eddy currents be as perpendicular to defects as possible to obtain maximum response. If eddy currents flow parallel to a defect, there will be little distortion of the eddy currents and hence little change in probe impedance.
Various eddy current probes have been proposed for inspecting cylindrical or tubular components as seen in U.S. Pat. Nos. 3,952,315 Apr. 20, 1976 (Cecco), 4,079,312 Mar. 14, 1978 (Osborne et al.) and 4,083,002 Apr. 4, 1978 (Allport).
A conventional internal circumferential probe induces a flow of eddy currents parallel to the coil windings and therefore circumferential in direction. As mentioned above, coil impedance must change to sense a defect. This will occur only if eddy current flow path is disturbed. Circumferential defects parallel to this current, which present no significant area perpendicular to this path, will therefore not be sensed. Multiple coils in excitation coil assembly and in receiver coil assembly are also described in U.S. Pat. Nos. 3,241,058 Mar. 15, 1966 (Quittner) and 3,271,662 Sept. 6, 1966 (Quittner). The above two patents to Quittner teach sheet metal inspection using an odd number of coils with their axes perpendicular to test sample for transmit and an even number of coils for receiving. The transmit coils are electromagnetically polarized alternately but the receiver coils are polarized in same directions, thus enabling no circumferential or line compensation (a desired feature in the Quittner patent). They are very complex and sensitive to probe wobble. They also produce complicated output signals to analyze and are not readily applicable for cylindrical testing.
U.S. Pat. No. 3,444,459 May 13, 1969 (Prindle et al.) describes helical sensing coils slightly skewed relative to the axis of the tube. The coils are alternately polarized but must be in an elongated shape for 100% circumferential coverage and manageable axial probe length. The transmit coil is at least three times larger than the sensing coil coil assembly. The probe has no sensitivity to circumferential cracks.
It is a recognized problem that reliable detection and sizing of circumferential cracks, fretting wear, shallow internal defects etc. is made more difficult by the fact that they frequently occur in defect prone regions such as under tubesheets or support plates and in transition region of finned tubes. Heat exchangers and steam generator are normally assembled with tubes rolled into the tubesheet and then welded at the primary tubesheet face. Rolling is primarily performed to eliminate corrosion prone crevices. However, if tubes are rolled beyond the tubesheet secondary face they are prone to cracking.
The present invention makes use of multiple coils operating in the transmit-receive mode for detecting all localized defects, including circumferential cracks in a tube. The probes according to the present invention detect internally or externally any defects including defects under support plates. With a variety of coil configurations, a 100% circumferential coverage in single pass is possible with high defect sensitivity in the transition zone between expanded and unexpanded tubing.